Stabilizer device for bottom hole assembly

ABSTRACT

A stabilizer device ( 20, 120, 220 ) for a drill stem, in particular for a bottom hole assembly, which is rotatable about a longitudinal axis (X), the device comprising a tubular central body ( 21 ) and connection means ( 22, 23 ) at its axial ends for connecting to elements of the stem, the device being provided with a helical protrusion ( 24, 25, 26 ) on the surface of the body intended to come into contact with a wall of the drilled hole, said helical protrusion turning in the clockwise direction (W) about the axis of the central body viewed from an upstream axial end in the direction of a downstream axial end, characterized in that a crest ( 33, 33′, 33″ ) of the helical protrusion comprises a leading edge ( 35, 35′  and  35″ ) and a trailing edge ( 36, 36′  and  36″ ) defined in the direction of rotation, the leading edge comprising a convex portion such that a first portion ( 38 ) of said convex portion has a radius of curvature of more than 3.5 mm over an angular arc of at least 20°.

The invention relates in general to a stabilizer for a bottom hole assembly. In the field of exploration and research into oil fields, strings of rotary drill pipe strings constituted by pipes and possibly other tubular elements are used which are connected end to end as required by the drilling conditions. A drill pipe string is a part of a drill stem which is tasked with drilling a well bore and which is composed of a lot of equipment for drilling. That pipe string is connected to the bottom hole assembly; together, they form a drill stem.

A drill stem is subjected to many loads, such as rotational and translational movements imposed by the surface equipment, vibrations created by the bottom hole tools and contacts and forces exerted between the well walls and the components of the stem; the well may be more than 12 km long.

A bottom hole assembly may be composed, in succession from the drill bit which is the element which is closest to the well bottom, of the drill bit, any components comprising the motors for the drill bit, drill collars, equipment known as MWD and LWD, stabilizer devices and heavy weight drill pipe strings.

In a drill stem in particular, there is a need for stabilizing the bottom hole assembly at several positions between the drill bit and the heavy weight pipes. A stabilizer acts to control the deformation of the bottom hole assembly, in particular by providing a pre-set point of inflexion for the bottom hole pipes and the drill bit. It also assists in limiting vibration in the pipe string and controls the deformation as an element contributing to controlling the deviation of the desired trajectory.

The invention may also be applied to components for measuring or inspecting the well bottom, such as equipment known in the art by the terms MWB, LWD or even RSS.

Conventionally, in order to allow drilling, pressurized mud passes over the drill bit disposed at the end of a string of drill pipe strings at a regulated flow rate inside the string of drill pipe strings and is then pumped and lifted in the annular space defined between the stem and the hole wall.

Various stabilizer tools are known in the art, such as those produced by the Applicant. Documents U.S. Pat. No. 4,275,935; U.S. Pat. No. 6,202,769 and US-2010-0300760 in particular disclose stabilizers provided with external helical protrusions attached to a tubular body or integral with said body.

The teaching of document US-2010-0300760 also discloses a set of tubular bottom hole drilling elements comprising a bore former disposed between the drill bit and three upstream stabilizers to improve the stability of the drill bit and the directional control of the trajectory.

The aim of the invention is to improve the consolidation of the hole walls while reducing the coefficients of friction with those walls. In fact, formation of a hole results in the formation of debris corresponding to the material which is torn out to form the hole. This debris is evacuated by lifting it out with the drilling mud. It turns out to be very important that lifting this debris does not modify the dimensional characteristics of the hole being formed.

Further, given the short service life of the drill bit at the hole bottom, when drilling a well with a length of 4 to more than 10 km, it is necessary to have to lift the drill stem assembly regularly in order to be able to change or repair the drill bit and then to drop the whole drill stem down again in order to progress hole formation a little further. These operations of lifting and dropping the string solely for the purposes of maintenance and analysis (known as tripping out and tripping in) give rise to many frictional loads on the hole walls and also make a substantial contribution to deterioration of the quality of the hole in terms of homogeneity of the diameter over its length, cracking risks and risks of causing mud deceleration and turbulence zones in enlarged diameter zones and thus to the accumulation of rubble. More generally, these operations, although necessary, contribute to weakening of the hole and thus to increasing the risk of the string getting blocked in the hole.

This problem becomes more and more critical when approaching the bottom of the well where the diameter of the hole is very close to that of the bottom hole assembly.

The aim of the present invention is to propose a technical solution in particular to limit damage to the walls of the hole during tripping out and tripping in operations while allowing proper circulation of the mud.

The invention provides a stabilizer device for a drill stem, in particular for a bottom hole assembly, which is rotatable about a longitudinal axis, the device comprising a tubular central body and connection means at its axial ends for connecting to elements of the stem, the device being provided with a helical protrusion on the surface of the body intended to come into contact with a wall of the drilled hole, said helical protrusion turning in the clockwise direction about the axis of the central body viewed from an upstream axial end in the direction of a downstream axial end, characterized in that a crest of the helical protrusion comprises a leading edge and a trailing edge defined in the direction of rotation, the leading edge comprising a convex portion such that a first part of said convex portion has a radius of curvature of more than 3.5 mm over an angular arc of at least 20°.

The term “helical protrusion” means a shape with an envelope which is generally helical.

As an example, this first part of the convex portion may have a radius of curvature of more than 5 mm over an angular arc of at least 30°.

The invention can be used to improve the geometric and mechanical coupling between the protrusions of the stabilizer and the walls of the well, in particular by limiting dynamic impacts which are the primary causes of lateral and torsional vibrations, and also by improving the homogeneity of the profile of the section of the hole being formed. The invention can also be used to obtain fluid bearing at the level of the stabilizer device.

The very particular shape of the leading edge provides for activated circulation of the drilling mud while providing a less aggressive profile as regards the walls of the hole being formed. Such a dimensional selection means that enlargement of the hole during insertion or removal operations for the drill string can be limited. This advantage is also obtained during rotation during drilling.

Advantageously, the convex portion may also be complex and comprise at least one second convex portion adjacent to the first portion, said second portion having a radius of curvature which is smaller than that of the first portion, said second portion being located ahead of the first portion in the direction of rotation.

As an example, the convex portion may extend over an angular arc of less than 180° and comprise a third portion, such that the second portion is disposed between the first and the third portion, and such that said third portion has a radius of curvature which is larger than that of the first portion. Such a configuration means that a larger cross section for movement of the drilling mud can be provided while improving contact, centring and guidance of the stem.

In particular, the parts of the convex portion may be linked tangentially. Such a configuration improves the dynamics of the rising drilling mud and the flow is homogeneous along the convex portion.

Advantageously, the crest of the helical protrusion can form an arc of a circle linked tangentially to the convex portion of the leading edge. In this manner, no edge is formed that can tear up the walls of the hole. Similarly, the crest of the helical protrusion can form an arc of a circle linked tangentially to the convex portion of the trailing edge.

In particular, the circular arc may have a diameter which is determined as a function of the theoretical diameter of the drilled hole at the well bottom or the external diameter of the drill bit at the well bottom; in particular, this diameter is determined by reducing the theoretical diameter by a value of at least 1/64 inch, i.e. 0.4 mm. This dimensional selection can be used to consolidate the walls of the hole by allowing a slight deflectional play at the stabilizer device.

The trailing edge may also comprise a second convex portion such that the crest of the helical protrusion forms a circular arc linked tangentially to this second convex portion.

Preferably, the device may comprise a material covering the crest and in part at least one of the leading edge and the trailing edge such that the hardness of this material is much greater than that of the helical protrusion. The presence of such a material means that wear of the portions of the protrusion which are most exposed to contacts and to shocks against the walls of the hole being formed can be limited, and thus the number of maintenance operations on the bottom hole elements can be limited.

In a particular embodiment, a housing may be formed at the crest of the helical protrusion in order to retain a rotationally-free roller. The invention also concerns the “roller reamer” category, namely tools provided with blades or rollers that are generally used to regularize and calibrate the walls of a well.

In particular, the helical protrusion can form a radial foot with a minimum width measured perpendicular to a bisecting line such that this minimum width is smaller than the maximum width of said foot closer to the crest than the minimum width. Such a configuration can be used to increase the available section for circulation of the drilling mud while maintaining the quality of contact between the hole and said stabilizer.

Advantageously, the device may comprise a concave zone between a leading edge and an adjacent trailing edge. Preferably, each concave zone may comprise a first concave portion linked tangentially to the leading edge, said first concave portion having a radius of curvature which is smaller than that of a second concave portion linked tangentially to the adjacent trailing edge. The flow of drilling mud is thus principally caused to circulate more under the leading edge than in the proximity of the trailing edge. Acceleration of the trailing edge can thus be improved in order to allow the drilling debris to be dislodged rapidly.

As an example, the concave zone may define a profile tangential to the outer perimeter of the tubular body. Such a configuration means that the central body is not weakened. Alternatively, the concave zone may be located short of the outer perimeter of the tubular body in order to increase the cross section of passage for the flow of mud between the protrusions.

In particular, the helical protrusions may be spaced by a developed distance such that two adjacent protrusions may overlap in less than one turn or even less than one half-turn, for example from a quarter turn.

Preferably, the helical protrusion turns through less than one turn about the body, in particular through a half turn. This configuration means that the presence of said protrusion or protrusions is limited axially such that the contact points of the protrusions with the walls of the hole being formed constitute the equivalents of points of inflexion for the remainder of the drill string. Advantageously, such a configuration can be used to improve the dimensioning of the hole and prevent it from being enlarged.

Preferably, the device may comprise protrusions over a total length defined between its axial ends of less than 120 inches.

Advantageously, the helical protrusion may be centred between the two axial ends of the device. Such a configuration means that the bending forces which may be applied to the device can be distributed in a balanced manner. Other non-centred configurations are also acceptable.

In another embodiment, the downstream axial end may be directly part of or, in a variation, adjacent and screwed onto, the drill bit (“near-bit” stabilizer configuration). In such a configuration, the stabilizer device is directly integrated into the drill bit; in particular, it is disposed downstream of the rotary motors in the zone which is generally termed the bit gauge.

In a variation, the tubular body may constitute a fixed envelope screwed around and onto a tubular component having said connection means. In this case, the invention is applicable to the element known as the sleeve stabilizer.

In a further variation, the helical protrusion and the tubular body may be produced in an integral manner.

Preferably, the device is produced from steel, for example from one of the steels corresponding to the following standards: AISI 4135; AISI 4137; AISI 4140; and/or AISI 4145. The steel may in particular be a magnetic.

In a particular embodiment, the central body may have a bore such that the minimum thickness of the wall of this central body represents more than 25% of the external diameter of this body. Such a configuration increases the solidity of the whole of the device.

The present invention will be better understood from the following detailed description of several embodiments which are given by way of non-limiting examples and illustrated in the accompanying drawings in which:

FIG. 1 illustrates an embodiment of a bottom hole assembly of a conventional drill stem in a hole being formed;

FIG. 2 diagrammatically illustrates the phenomenon of partial collapse of walls of the drilled hole;

FIG. 3 is a perspective view of a first embodiment of a stabilizer device of the invention;

FIG. 4 is a longitudinal view of a blank necessary for the production of a device in accordance with FIG. 3;

FIG. 5 is a longitudinal view of FIG. 3;

FIG. 6 is a cross sectional view in the sectional plane A-A indicated in FIG. 5;

FIG. 7 is an enlarged partial cross sectional view of FIG. 6;

FIG. 8 is a cross sectional view in the sectional plane B-B indicated in FIG. 5;

FIG. 9 is a developed diagrammatic view of the helices formed by the protrusions of a device of the invention;

FIG. 10 is a distal view from a downstream male end of a device of the invention;

FIG. 11 is a partial top profile view of a stabilizer device in accordance with a variation of the invention;

FIG. 12 is a cross sectional view in the sectional plane C-C indicated in FIG. 11;

FIG. 13 is a cross sectional view in the sectional plane D-D indicated in FIG. 11;

FIG. 14 is a variation of the embodiment of FIG. 13;

FIG. 15 is a profile perspective view of a stabilizer device in accordance with another variation of the invention;

FIG. 16 is a cross sectional view in the sectional plane E-E of FIG. 15.

FIG. 1 shows an embodiment of a bottom hole assembly 1. This assembly 1 comprises a plurality of tubular components associated end to end forming a bottom hole assembly upstream of the drill bit 2 disposed at one axial end of said stem. For drilling reasons, this assembly 1 is driven in rotation in the clockwise direction, considered from an upstream end in the direction of a downstream end constituted by the drill bit. In drilling technology, the direction of rotation is always determined from the surface relative to the well bottom.

The drill bit comprises a motorized distal end 3 which is intended to excavate the hole, and an upstream portion 4 supporting the motorized end 3. The upstream portion 4 is occasionally termed a bit gauge. In the example shown, the upstream portion 4 is smooth.

The upstream portion 4 is then connected by makeup to a first tubular component 5 which is then connected upstream, by makeup, to a first stabilizer device 6 of the invention. This stabilizer device 6 is itself made up upstream to a well bottom measuring device 7. This well bottom measuring device 7 may comprise several tubular sections made up end to end in order to carry out a plurality of measurements and checks at the well bottom. This measuring device 7 may also comprise rotary-steerable means (RSS) for the drill bit 2.

This measuring device 7 is connected upstream to a second stabilizer device 8 of the invention, which may be structurally different from the first stabilizer device 6. Preferably, the second stabilizer device 8 has been connected to thick-walled tubular elements termed a drill collar both downstream and upstream.

The second stabilizer device 8 is connected upstream to a calibration tool 9 provided with blades or rollers that are free to rotate at its walls and are intended to come into contact with the hole walls. In the example shown, the calibration tool has an adjustable diameter. Alternatively, at the position of this calibration tool 9, it is possible to provide a roller reamer the external diameter of which is not adjustable but which could form a third stabilizer device with freely rotatable rollers, as will be described in detail below.

This calibration tool 9 is itself connected upstream to a fourth stabilizer device 10 in accordance with the invention, which may be different in shape, structure and/or dimensions compared with the preceding stabilizer devices disposed downstream. This fourth stabilizer device 10 is connected to a group 11 of several tubular components in particular comprising a sliding component 12 that is capable of expanding axially on command in order, for example, to unblock the bottom hole assembly. This sliding component 12 is generally known as a jar.

This group 11 is then made up upstream with one or more heavy pipes 13 generally known as heavy weight drill pipes before being connected to a drill string principally constituted by drill pipe.

As can be seen from this overall description, a stabilizer device in accordance with the invention may be placed at several positions in a bottom hole assembly 1 and have mutually differing shapes, structures and/or dimensions.

These stabilizer devices have, at least locally along their longitudinal axis, an external diameter close to that of the hole being formed or the external diameter of the drill bit. Their role is to keep the pipe string and the bottom hole assembly stable in the hole while the drill bit is subjected to very powerful vibrations in contact with the rock to be excavated, while keeping the pipe string rotating in the hole. In addition, these stabilizer devices contribute to improving directional control of the drill bit in the formation to be excavated in order to reach hydrocarbon-bearing rocks which will then be worked.

FIG. 2 illustrates a classic drilling problem. Because geological strata are traversed which have different hardnesses and even different structures, the hole being formed may pass through different strata. The quality of the walls of the hole and their continuity is a parameter which is very important to control during drilling in order to completely control the rising mud flows and also to prevent the drill string from being blocked in situ.

FIG. 2 shows an example of the partial collapse of the walls of a hole being formed in a friable zone 14. The invention can be used to limit erosion and weakening of the hole walls. These fallen rocks 15 of randomized sizes fall down to the bottom of the well and risk damaging the drill bit per se, even more so when it is not designed to crush this type of fallen rocks into pieces with a size similar to those produced during drilling. As can be seen in this FIG. 2, there is a genuine need for consolidation of the walls of the hole being formed without damaging them and of limiting the vibrations which can favour fallen rocks.

FIG. 3 presents a first embodiment of a stabilizer device 20 of the invention. The stabilizer device 20 comprises a tubular central body 21 with a longitudinal axis X and is provided at these axial ends with threaded portions respectively 22 and 23 to enable it to be connected to other elements of the bottom hole assembly. In the example shown, the end 22 comprises a male connector with a visible threading, while the other end 23 has a female connector, the threading of which, which is not shown, is located on the inner perimeter of this end.

The outer perimeter of the central body 21 is generally a regular circle in section transverse to the axis X. At the position where three radial helical protrusions 24, 25, 26 extend beyond the outer perimeter of the central body 21, the device 20 locally has an envelope surface the cross section of which has a diameter which is greater than that of the central body 21. These radial protrusions are generally termed blades.

The three protrusions turn about the axis X. They each turn through a half turn about the axis X in the example shown. The respective starting points of each of these helical protrusions are distributed regularly about the periphery of the central body 21.

Assuming that the male connector can constitute a downstream connector of this device relative to the drill string, it may be considered that the helical protrusions turn in the clockwise direction considered from an upstream end in the direction of a downstream end. These protrusions turn in the same direction as the direction of rotation W of the pipe string and thus of the bottom hole assembly in the hole.

A tubular blank E such as shown in FIG. 4 is used as the starting point for producing such a stabilizer device 20. This tubular blank E is preferably produced as a single piece in a single material which is then machined to size. In practice, this blank E already comprises the central body 21 at the ends of which the respective male 22 and female 23 connectors will be machined. The blank E comprises a median portion 27 substantially at equal distances from its axial ends 22 and 23. The distance between the ends 22 and 23 is, for example, 120 inches or less. The median portion 27 forms a tubular portion with an external diameter which is greater than that of the rest of the central body.

In particular, the internal diameter of the bore of the blank E may be constant over its entire length. As an example, the thickness of the wall of such a stabilizer device 20 is selected such that it represents more than 25% of the external diameter, namely the diameter OD1 and/or the diameter OD2.

The variation in diameter between the diameter OD1 of the central body 21 and the diameter OD2 of the median portion 27 is approximately symmetrical in this example either side of the median portion 27. In a variation, not shown, the variations in upstream and downstream diameters may be asymmetrical.

In particular, this enlargement 32 comprises, in succession from the diameter OD1:

-   -   a flared concave portion 28 with a radius of curvature R1, for         example of the order of 85 mm, in the range 50 to 300 mm;     -   followed by a flared planar portion 29 with an inclination, for         example, of the order of 45° relative to the axis X, over a         distance of the order of 100 mm;     -   followed by a convex first fillet portion 30 with a radius of         curvature R2, for example of the order of 80 mm, in the range 50         to 300 mm;     -   followed by a second convex fillet portion 31 with a radius of         curvature R3 which is greater than the radius of curvature R2,         for example of the order of 130 mm, in the range 50 to 300 mm;     -   this second convex fillet portion 31 connecting to the outer         perimeter with external diameter OD2.

In the example shown, each portion of the enlargement 32 links tangentially to the next. Similarly, the central body 21 links tangentially to the concave flared portion 28. In addition, the convex fillet portion 31 is linked tangentially to the outer perimeter of the median portion 27.

This progression of the enlargement 32 contributes to softening contact between the protrusions 24, 25 and 26 with the walls of a hole being drilled. This prevents scraping or penetration into the hole wall. This progression is retained in the ends which begin and terminate the various protrusions 24, 25 and 26. In fact, as can be seen in FIG. 5, the protrusions 24, 25 and 26 are formed in the thickness of these enlargements 32 and in the median portion 27. Thus, as the drill string is advanced and/or withdrawn, the protrusions of a stabilizer device of the invention come into progressive contact with the hole wall and no not excavate it any more than has already been done.

The cross sectional views of FIGS. 6 and 7 provide a better understanding of the dimensions of the protrusions machined in the blank E.

As can be seen in FIG. 6, the protrusions 24, 25 and 26 are distributed angularly at the crests of an equilateral triangle; its bisecting lines B, B′ and B″ are shown. The three protrusions are superimposable in shape. Each protrusion 24, 25 and 26 respectively has a crest 33, 33′ and 33″ forming a non-machined portion of the enlargement 32 or of the median portion 27. Each crest 33, 33′ and 33″ covers an angular arc 34 of the order of 32° when it is formed in the median portion 27, preferably in the range 20° to 60°. This angular arc 34 is the same or smaller when it is evaluated in the enlargement 32. Each crest 33, 33′ and 33″ is preferably distributed symmetrically either side of the bisecting line with which it intersects, respectively B, B′ and B″. Alternatively, the crests could respectively be distributed in a non-symmetrical manner either side of the bisecting lines.

In operation, as the drill string advances in the hole, the stabilizer device 20 is driven in rotation in the clockwise direction W. Each protrusion 24, 25, 26 respectively has a leading edge 35, 35′ and 35″ and a trailing edge 36, 36′ and 36″ relative to this clockwise direction. The leading edge is the edge which will come into contact first with the wall of the hole, and the trailing edge will come into contact next.

The leading edges 35, 35′ and 35″ and the trailing edges 36, 36′ and 36″ each comprise a convex portion. In the example shown, the leading edges and the trailing edges are solely constituted by said convex portion. The leading edges form a complex convex portion which is generally not superimposable, nor symmetrical with the complex convex portion constituted by the trailing edges. Alternatively, the leading edges could be designed so as to be symmetrical with the trailing edges. In this example, the leading edges 35, 35′ and 35″ are mutually superimposable. Similarly, the trailing edges 36, 36′ and 36″ are mutually superimposable.

In particular, the leading edge 35 is linked to the trailing edge 36″ via a first concave zone 37. Similarly, the leading edge 35′ is linked to the trailing edge 36 via a second concave zone 37′. And the leading edge 35″ is linked to the trailing edge 36′ via a third concave zone 37″. The leading edge 35, together with a portion of the concave zone 37, the crest 33, the trailing edge 36 and a portion of the concave zone 37′ form a radial foot relative to the central body 21. In particular, the foot or base of the blade on the body has a minimum width Lmin at a non-zero distance from the crest, this minimum width Lmin being evaluated perpendicular to the bisecting line B. In particular, the foot comprises a maximum width Lmax at a position located between the crest and the minimum width Lmin. The position of the minimum width Lmin is located at a tangent to the outer perimeter of the central body 21 or slightly beyond it in the direction of the crest 33.

In detail, FIG. 7 shows an enlargement of the leading edge 35 of the first concave zone 37 and of the trailing edge 36″.

The leading edge is, for example, constituted by three successive convex portions 38, 39 and 40. These portions 38, 39 and 40 are linked tangentially. The second portion is disposed between the first and the third portions. The first portion 38 is linked tangentially to the crest 33 with a radius of curvature OD2.

In the case in which the diameter OD2 is 311.15 mm (12¼ inches), then, the radius of curvature of the first portion 38 is of the order of 39 mm and extends along an angular arc of the order of 30°.

The table below illustrates the possible dimensional choices as a function of the diameter of the drilled hole and thus as a function of the maximum diameter OD2 observed at the crests of the stabilizer device 20.

Max diameter of the Maximum Minimum radius drilled hole at the well diameter of crest of curvature of first bottom (D), in inches (OD2), in inches portion 38, in mm  6 D-1/64  3.5 mm (9/64 inch)  8 ½ D-1/64  4.3 mm (11/64 inch) 12 ¼ D-1/32  7.5 mm (19/64 inch) 17 ½ D-1/16 13.9 mm (35/64 inch) 26 D-1/16 13.9 mm (35/64 inch)

Adjacent to the first portion 38, the second portion 39 has a radius of curvature which is smaller than that of the first portion. It also covers an angular arc that is smaller than the first portion. In particular, for the described embodiment in which the diameter OD2 is 12¼ inches, a radius of curvature of 25 mm is used for this second portion and for an angular arc of the order of 15°.

Adjacent to the second portion 39, the third portion 40 has a radius of curvature which is higher than that of the first portion and also higher than that of the second portion. This third portion 40 covers an angular arc which is greater than that of the second portion and nevertheless smaller than the first portion. In particular, for the described embodiment in which the diameter OD2 is 12¼ inches, a radius of curvature of 46 mm is used for this third portion 40 and for an angular arc of the order of 20°.

Advantageously, the set of the three portions 38, 39 and 40, namely the complex convex portion 35 of the leading edge 38, are circumscribed by a single convex portion covering an angular arc of more than 90° and less than 180° such that this circumscribed single convex portion has a radius of curvature equal to the largest of the individual radii of curvature of each of the portions constituting it.

In the example of FIG. 7, the trailing edge 36″ is, for example, constituted by two successive convex portions 41 and 42 in order to form a complex convex portion. These portions 41 and 42 are linked tangentially. The fourth portion 41 is linked tangentially to the crest 33″ with radius of curvature OD2.

In the case in which the diameter OD2 is 12¼ inch, then, the radius of curvature of the fourth portion 41 is of the order of 25 mm and extends in an angular arc of the order of 25°. This fourth convex portion 41 is disposed between the crest 33″ and the fifth portion 42. The fifth portion 42 has a radius of curvature which is greater than that of the fourth portion 41. It also covers an angular arc which is greater than or equal to that of the fourth portion 41. In particular, for the embodiment described in which the diameter OD2 is 12¼ inch, a radius of curvature of 36 mm is used for this fifth portion 42 and over an angular arc of the order of 50°.

Advantageously, the set of the two portions 41 and 42 form a complex convex portion of the trailing edge 36″ circumscribed by a simple convex portion covering an angular arc of more than 90° and less than 120°, such that this circumscribed simple convex portion has a radius of curvature equal to the largest of the individual radii of curvature of each of the portions constituting it.

As can be seen in FIG. 7, the leading edge 35 is linked to the adjacent trailing edge 36″ via the concave zone 37. In practice, this concave zone 37 comprises a tangent point 43 with an imaginary circle with a diameter less than or equal to the value OD1, and corresponding to the external diameter of the central body 21.

Between the leading edge 35 and the tangent point 43, that portion of the concave zone 37 belongs to the foot formed by the first protrusion 24. Between the trailing edge 36″ and the tangent point 43, the other portion of the concave zone 37 belongs to the foot formed by the third protrusion 26. This other portion comprises a second concave portion 45 adjacent to the trailing edge 36″.

The radius of curvature of the first concave portion 44 is less than that of the second concave portion 45. In practice, in the example of FIG. 7, the radius of curvature of the first concave portion 44 is of the order of 21 mm, while that of the second concave portion 45 is of the order of 68 mm. The angular arc covered by each of these concave portions 44 and 45 is in the range 15° to 40°, for example of the order of 25°.

In the embodiment described, the concave zone 37 is in practice a complex concave surface. It is composed, in succession from the adjacent first concave portion 44, of a third concave portion 46 extending with the same radius of curvature to a tangent point 43. The third concave portion 46 is tangentially linked to a fourth concave portion 47 extending with the same radius of curvature to the second concave portion 45.

The radius of curvature of the third concave portion 46 is larger than the radius of the three other concave portions 44, 45 and 47. In practice, in the example of FIG. 7, the radius of curvature of the third concave portion 46 is of the order of 100 mm while that of the fourth concave portion 47 is of the order of 50 mm. The angular arc covered by each of these concave portions 44 and 45 is in the range 15° to 40°.

The angular arc covered by each of these concave portions 44, 45, 46 and 47 is in the range 80° to 100°.

After having described the section of the device 20 in detail by means of FIGS. 6 and 7 in a zone in which the protrusions have their crest with the same radius of curvature as a circle with diameter OD2, we shall now described the details of FIG. 8, corresponding to a section of the protrusions observed in an upward incline constituted by one of the enlargements 32.

The distinctions between the section of FIG. 6 and that of FIG. 8 will now be highlighted:

-   -   in the enlargement zone, the radial foot formed by each         protrusion does not comprise a maximum width Lmax at a position         located between the crest and the minimum width Lmin. In fact,         the width of each foot decreases continuously from its zone of         attachment to the central body 21 to its crest 33;     -   the leading edges 35, 35′ and 35″ and the trailing edges 36, 36′         and 36″ extend over an angular arc which is much smaller than         the respective angular arcs described above for FIGS. 6 and 7;     -   the leading edges 35, 35′ and 35″ and the trailing edges 36, 36′         and 36″ may be convex surfaces with a continuous radius of         curvature in the enlargements 32;     -   the trailing edges 36, 36′ and 36″ have a radius of curvature         which is generally less than or equal to that of the leading         edges 35, 35′ and 35″;     -   the concave zones 37 may be concave surfaces with a continuous         radius of curvature in the enlargements 32.

The set of dimensions given above are measured after depositing a layer of a highly friction-resistant material. In fact, given the envisaged use of such stabilizers, it is usual to cover at least the crests and at least a part of the leading edges and trailing edges with a material which is generally termed a hardbanding material. This material may be deposited by welding, for example by laser welding, or by spraying or surface treatment processes.

In the embodiment shown diagrammatically in FIG. 9, the blades of the protrusions are spaced by a developed distance d which is very substantially smaller than the axial distance covered by the blade in one turn. A geometrical pitch is the distance covered by the blade in making one turn. In practice, the blades of the protrusions 24, 25 and 26 make less than one turn, and rather, substantially half a turn.

In practice, the developed distance d is less than the axial distance actually traversed by the protrusion, Dp. As an example, the developed distance d is of the order of the maximum width Lmax of a protrusion.

In a variation, not shown, the blade may turn about the principal body with a non-constant angle, for example gradually increasing along the blade.

FIG. 10 shows that the maximum width, Lmax, of a protrusion increases between its beginning in the enlargement 32 to the median portion 27, this variation possibly resulting from the presence of a transition portion between the tubular central body 21 and the median portion 27, in which portion the protrusions 24, 25 and 26 begin and in which the leading edges and trailing edges may comprise other radii of curvature than those described above, in order to prevent said edges from excavating the hole walls being formed, in particular under the effect of advance or withdrawal of a drilling component in the well.

FIG. 11 shows a variation of a stabilizer device in accordance with the invention. In the example of FIG. 11, the stabilizing function of the device 120 is carried directly in a downstream portion termed the bit gauge at the drill bit, not shown. In this embodiment, the device 120 embodying the invention is disposed at one end of the bottom hole assembly, this latter comprising a single connector 122 for connection with the upstream elements.

In a zone 150, defined axially upstream of the drill bit, close to the connector 122, the outer perimeter has protrusions with a profile corresponding to that observed in the first embodiment of the stabilizer 20. The distinction between the stabilizer 20 arises from the fact that the zone 150 in this example 5 comprises protrusions distributed evenly over the perimeter of this zone.

Downstream of this zone 150 in the direction of the well bottom, the device 120 comprises a zone 151 with a cross section which may have two alternatives, shown respectively in FIGS. 13 and 14. In FIGS. 13 and 14, each of the protrusions of the zone 150 continues with the same pitch as in the zone 151, but changes in cross section. In fact, in FIG. 13, each leading edge of the five protrusions has an acute angle in order to excavate, in part, the walls of the hole being formed. When the protrusions are at an acute angle, their leading edges may include inserts formed from polycrystalline diamond (PCD).

In a variation, in FIG. 14, only three protrusions out of the six have leading edges with such sharp angles. The protrusions with a profile of the type seen in zone 150 are alternated with the protrusions with an sharp profile.

In particular, as can be seen in FIG. 11, in the zone 151, successively along the axis X, the leading edge may have profiles which are sharp to a greater or lesser extent. In particular, it may have a repeat pattern which alternates softened profiles and sharp profiles.

The device of the invention may also be integrated into a portion of a drill bit known as the bit gauge.

In yet another variation of the invention, shown in FIGS. 15 and 16, a roller reamer 220 is described which has profiles for its leading edges and trailing edges which are identical to those described in the embodiment shown in FIGS. 2 to 7. In contrast, in this example, the protrusions are such that the blade has a twist with a zero angle of inclination relative to the longitudinal axis X. The difference between this embodiment and those described above also derives from the fact that the respective crests 33, 33′ and 33″ are each provided with a housing 250 which opens radially outwardly. This housing 250 holds a roller 251. In particular, the roller 251 is provided with pins to improve calibration of the walls of the well. In general, these pins are produced from tungsten carbide.

Throughout the description, the expression “comprising a” should be construed as being synonymous with “comprising at least one”, unless specifically stated otherwise. 

1. A stabilizer device (20, 120, 220) for a drill stem, in particular for a bottom hole assembly, which is rotatable about a longitudinal axis (X), the device comprising a tubular central body (21) and connection means (22, 23) at its axial ends for connecting to elements of the stem, the device being provided with a helical protrusion (24, 25, 26) on the surface of the body intended to come into contact with a wall of the drilled hole, said helical protrusion turning in the clockwise direction (W) about the axis of the central body viewed from an upstream axial end in the direction of a downstream axial end, characterized in that a crest (33, 33′, 33″) of the helical protrusion comprises a leading edge (35, 35′ and 35″) and a trailing edge (36, 36′ and 36″) defined in the direction of rotation, the leading edge comprising a convex portion such that a first portion (38) of said convex portion has a radius of curvature of more than 3.5 mm over an angular arc of at least 20°.
 2. A stabilizer device according to claim 1, characterized in that the convex portion is complex and comprises at least one second convex portion (39) adjacent to the first portion, said second portion having a radius of curvature which is smaller than that of the first portion, said second portion being located ahead of the first portion in the direction of rotation.
 3. A stabilizer device according to claim 2, characterized in that the convex portion extends over an angular arc of less than 180° and comprises a third portion, such that the second portion is disposed between the first and the third portion (40), such that said third portion has a radius of curvature which is greater than that of the first portion.
 4. A stabilizer device according to claim 1, characterized in that the parts of the convex portion are linked tangentially.
 5. A stabilizer device according to claim 1, characterized in that the crest of the helical protrusion forms an arc of a circle linked tangentially to the convex portion of the leading edge.
 6. A stabilizer device according to claim 5, characterized in that the circular arc has a diameter (OD2) which is determined as a function of the theoretical diameter of the drilled hole at the well bottom; in particular, this diameter is determined by reducing the theoretical diameter by a minimum value of 1/64 inch.
 7. A stabilizer device according to claim 1, characterized in that the trailing edge comprises a second convex portion (41, 42) such that the crest of the helical protrusion forms a circular arc linked tangentially to this second convex portion.
 8. A stabilizer device according to claim 1, characterized in that it comprises a material with a substantially greater hardness than that of the helical protrusion, this material covering the crest and part of at least one of the leading edge and the trailing edge.
 9. A stabilizer device according to claim 1, characterized in that a housing (250) is formed at the crest of the helical protrusion in order to retain a roller which is free in rotation (251).
 10. A stabilizer device according to claim 1, characterized in that the helical protrusion forms a radial foot with a minimum width (Lmin) measured perpendicular to a bisecting line (B) such that this minimum width is smaller than the maximum width (Lmax) of said foot closer to the crest than the minimum width.
 11. A stabilizer device according to claim 1, characterized in that it comprises a concave zone (37, 37′, 37″) between a leading edge and a trailing edge.
 12. A stabilizer device according to claim 11, characterized in that each concave zone comprises a first concave portion (44) linked tangentially to the leading edge, said first concave portion having a radius of curvature which is smaller than that of a second concave portion (45) linked tangentially to the adjacent trailing edge.
 13. A stabilizer device according to claim 12, characterized in that the helical protrusions are spaced by a developed distance (d) which is less than the axial distance covered by the helix.
 14. A stabilizer device according to claim 1, characterized in that the helical protrusion turns through less than one turn about the body, in particular over a half turn.
 15. A stabilizer device according to claim 1, characterized in that the downstream axial end is attached directly to the drill bit.
 16. A stabilizer device according to claim 1, characterized in that the helical protrusion and the tubular body are produced in an integral manner.
 17. A stabilizer device according to claim 1, characterized in that the central body has a bore such that the minimum thickness of the wall of said central body represents more than 25% of the external diameter of said body. 